Various techniques have been used in the past to achieve noninvasive imaging of blood flow using ultrasound. Recent developments in Doppler echocardiography are an example. Although the present invention is applicable to other uses, it will be described below in connection with its applicability to Doppler ultrasound blood flow imaging.
A typical ultrasound blood flow imaging system includes an ultrasonic transmit-receive transducer for transmitting ultrasonic pulses into a region of the body under diagnosis and for receiving echo signals of the transmitted ultrasound waves reflected due to blood flow in the area being scanned. One type of transducer is in the form of a probe containing a curved linear array of piezoelectric elements that insonify a sector shaped area of the body. A typical diagnosis with ultrasound includes scanning the patient with the ultrasound probe to measure blood flow in an artery, a vein, or in the heart. A signal processing system processes the received echo signals for measuring the Doppler shift frequency of the echo signals to thereby calculate the velocity of the blood flow, and the result of the velocity distribution measurement is displayed as a Doppler blood flow image. Techniques have been conventionally used for displaying the Doppler shift as a black and white image displaying velocity (B-mode gray scale display of echo amplitudes); in more recent years, color imaging techniques have been developed for displaying the two dimensional velocity distribution of blood flow in the area under diagnosis.
In order to estimate the Doppler shifts of the echoes received from the blood cells, an ultrasonic imaging system commonly transmits several (e.g. 4-16) pulses at one angle in the region under diagnosis and then detects the variations in the phase of the echoes from pulse to pulse.
Echo signal components Doppler-shifted by the blood flow are extracted from the Doppler signal components carrying the information of the internal moving part of the body. Echo signal components reflected from stationary targets, such as stationary tissue, are removed. These stationary signals are referred to as "clutter." They must be removed since their relative amplitude is typically orders of magnitude greater than the Doppler-shifted signals contained in the same data.
A stationary cancelling filter (also called a moving target indication filter or MTI) is used to eliminate clutter signals caused by stationary objects. In a typical MTI filter, echo signals from consecutive sound receptions are subtracted. Since the echoes from blood flow are superimposed on a much stronger tissue echo, the subtraction steps can, on the average, eliminate the signal due to stationary objects and extract the Doppler component representing blood flow. The MTI filter output is then processed by a velocity estimator to extract the Doppler frequency information, which is converted to velocity data signals suitable for display in color or on a B-mode gray scale display of echo amplitudes.
A principal object in the design and development of a Doppler color flow imaging system is to improve flow image quality, including slow flow sensitivity. These objectives can be achieved by signal processing techniques that result in higher Doppler signal-to-clutter ratios. It is also important to provide such improvements without adversely affecting system costs or image frame rates.
One limitation in the slow flow sensitivity of prior art systems arises from the use of the MTI filter, which has a frequency response which is not flat in the frequency range which includes the Doppler frequency signals representing blood flow. As a result of this frequency response characteristic, the MTI output signal contains a spectral bias which, when processed by the velocity estimator, produces errors in the blood flow estimate.